1. Field of the Invention
This invention relates to a pneumatic control system for use in conjunction with a controllable damper in an air handling system for automatically closing the damper in the event of a fire. More particularly, this invention relates to a three-way emergency relief valve for use in such pneumatic control system.
2. Description Of The Prior Art
It is known that it is desirable to close the blades or louvers of a damper, for example, a damper of the type used in a building air handling system, to the flow of air in the event of a fire, to thereby prevent the spread of smoke and high temperature gases throughout the building. Heretofore, this has been done, for example, by dampers of the type described in U.S. Pat. No. Re. 30,204 (Root) in which the damper blades are spring biased toward their closed positions and in which the mechanical blade operating linkage, which can normally operate to position the damper blades at open, flow permitting positions, includes a fusible portion. This portion frequently referred to as a fusible link, is designed to fail in an environment where the temperature exceeds a predetermined value. Thus, in the event of a fire in such a system, eventually the fusible link will overheat and fail, and the damper blades will be returned to their closed, flow restricting positions by the spring elements which normally bias the blades toward such positions. However, in a system of this type, a fusible link must be relatively large, since it is exposed to the full magnitude of the loads which are transmitted through the blade operating mechanism. Furthermore, because of the size of such fusible links and the thermal capacitance which results therefrom, it takes a significant amount of time for the fusible link to reach its failure temperature after the surrounding temperature reaches such level, and this results in a delay in the closing of the damper. U.S. Pat. No. 3,312,159 (W. Shepard) also discloses a prior art air handling system damper which will be automatically closed in the event of a fire as a result of the failure of a fusible link in its blade operating mechanism.